Christopher Kerr

The Electrodeposition of Composite Coatings based on Metal Matrix-Included Particle Deposits

SUMMARY Historically, the origins of composite (or inclusion) electrodeposition can be traced back to the early 1900s although the majority of modern developments can be considered to have taken place over the last 40 years. The increasing demands of industrial surface engineering have provided a driving force for rapid developments over the last decade. Co-deposition techniques can be used to produce a wide range of metal matrix-included particle coatings. The thickness of the overall coating can range from sub-micron to tens of microns while the included particles typically have sizes in the range 0.05-50 microns. The metal matrices (which include Ni. Co, Cu, Pb and Cr) can be deposited by electroplating or by electroless plating; particles range from hard materials (e.g., SiC, WC, Al2O3. CrC and BN), to self-lubricating ones (e.g., PTFE, C, MoS2 and encapsulated oils) and second metal powder phases. These deposits combine the advantages of the metal matrix and the included particles. For example, the metal matrix can confer high electrical and thermal conductivity while the type and degree of inclusion can tailored to the tribological properties required. Important examples include Ni/SiC deposits for wear resistance and Ni/PTFE ones for their self-lubricating and anti-stick characteristics. More ambitious composite coatings include those where a semi-continuous release of the included particles occurs under service conditions, e.g., slow release of oil particles or PTFE fragments. This paper provides a concise review of the field of composite electrodeposition and highlights the importance of process control in obtaining critical deposit characteristics for a variety of demanding industrial applications. In order to achieve high quality deposits, it is essential to control the electrolyte composition and process conditions (e.g., electrolyte flow conditions, solution pH and organic additive levels). Existing theories cannot adequately predict the deposit composition and properties from knowledge of bath composition and process conditions and further work in this area is essential. Recent developments in composite coating technology are profiled, including the emergence of compositionally- and hydrodynamically modulated layer coatings, the possibility of slow release coatings for semi-continuous lubrication and modification of diffusion coatings by heat treatment.

Porosity and Corrosion Rate Measurements for Electroless Nickel Deposits on Steel Using Electrochemical Techniques

SummaryElectrochemical techniques are described for the assessment of porosity in electroless nickel deposits on steel substrates from hypophosphite baths. Methods evaluated include the monitoring of the corrosion potential, determination of corrosion rate of coated samples (by a linear polarisation resistance technique, by Tafel extrapolation of polarisation plots and by the use of anodic polarisation curves) in 0.125M H2SO4 at 22°C. The shapes of the porosity vs. coating thickness curves were similar for all the methods investigated and the porosity increased markedly in the case of thinner deposits.

Studies of Porosity in Electroless Nickel Deposits on Ferrous Substrates

SummaryThe effect of soak and electrolytic alkaline cleaners together with chemical polishing on the porosity of electroless nickel coatings on mild steel substrates was investigated. The porosity of the deposit was evaluated by chemical spot tests using ammonium thiocyanate as indicator, the sulphur dioxide test, Corrodkote, electrochemical corrosion rate measurements and scanning electron microscopy studies. The results showed that the various pretreatments had no significant effect on the porosity of the coating. The overriding factor was observed to be the deposition time hence the thickness of the deposit. All the tests indicated that the percentage porosity decreased exponentially with increase in coating thickness. The relative merits of each of the techniques used in assessing the uniformity of the deposit, are discussed.

Monitoring of zincate pre-treatment of aluminium prior to electroless nickel plating

Zincating is used as a pre-treatment for aluminium prior to electroless nickel deposition during preparation of magnetic computer memory discs. Four immersion zincating solutions were evaluated at 22°C using single step or double zincating followed by electroless nickel deposition from a high phosphorus hypophosphite bath at 90°C. The coating process was monitored by potential vs. time curves obtained under open-circuit conditions during zincating then electroless nickel plating. The surface morphology of the aluminium, at various stages, was imaged by scanning electron microscopy and atomic force microscopy. Zero resistance ammetry was used to record galvanic currents between the aluminium and an inert platinum counter electrode during zincating. This, together with potential-time measurements, provided simple and valuable methods for following the zincating process and subsequent electroless Ni plating. Double zincating enabled a shorter induction time for electroless Ni deposition and resulted in a more complete coverage of the surface by Zn.

Electrolytic Deposition (Electroplating) of Metals

A. (a). Why do parts need to be electroplated?1-s The purpose of electrodepositing a coating is to confer different properties or dimensions to the article being plated. For example, the coated deposit may possess a greater corrosion resistance or lustre when compared to the base metal. Thus, the coating can be said to have functional or decorative properties. Changing the dimensions of a work piece can be especially useful if the part has been damaged during fabrication or in use, i.e., electroplating can salvage damaged or over-machined areas and, therefore, the work pieces can be salvaged instead of being scrapped. Other examples of this would be the use of hard chrome plating in the re-conditioning of automotive parts.